1. Field of the Invention
The invention relates to lead-free bismuth-containing silicate glasses. The invention also relates to uses of these glasses.
2. Prior Art
Television tubes operating by the cathode-ray principle consist of the glass parts picture screen (panel), funnel and neck. These glass parts enclose an evacuated space. The neck is that part of the tube, which contains one or more electron-beam guns. The panel includes, inter alia, one or more luminescent substances. The funnel is the conical part of the tube.
To meet the different requirements, the three above-mentioned glass parts have different physical properties as a result of different compositions.
The requirements placed upon funnel glass include high X-ray absorption, high electrical resistance and satisfactory melting and processing properties. In contrast to the panel glass, discoloration as a result of the electron radiation does not present a problem here. As the wall thickness of the funnel is lower than that of the panel, funnel glass must have a higher X-ray absorption coefficient xcexc. In conventional funnel glasses, xcexc as a value of xe2x89xa760 cmxe2x88x921 at 0.06 nm. This high value is usually achieved by high PbO concentrations in the glass.
The requirements placed upon neck glass are very similar to those placed upon funnel glass. As the neck wall is even thinner than the funnel wall, neck glass must have an even higher X-ray absorption coefficient xcexc: in conventional neck glasses, it is xcexcxe2x89xa790 cmxe2x88x921 (at 0.06 nm). Conventional prior art neck glasses likewise have a very high lead content.
Since the glass component PbO has recently come under public discussion as an environmental pollutant, manufacturers of devices containing cathode-ray tubes also have a need for PbO-free glasses having the requited physical properties.
Attempts to reproduce the physical and glass-technological properties effected by PbO by simple replacement of lead oxide by one or more constituents are generally unsuccessful. Instead, new developments in glass composition are necessary.
The patent literature already contains various documents, which describe glasses for cathode-ray tubes. However, these glasses have a variety of disadvantages:
JP 7-206468 A and JP 7-206471 A describe funnel glasses and neck glasses, respectively, for cathode-ray tubes which have a high Bi2O3 content and contain PbO at least as an optional component.
JP 9-142873 describes glasses for cathode ray tubes, which, in addition to a similar Bi2O3 content, contain CeO2 to prevent discoloration of the glass caused by the electron radiation.
It is an object of the invention to provide glasses which meet the above-mentioned requirements and which, in particular, have the necessary X-ray absorption. It is an object of the invention to provide glasses, which are suitable for use as funnel glasses.
It is another object of the invention to provide glasses, which are suitable for use as neck glasses.
It is an additional object of the invention to provide glasses which meet the requirements placed upon soldering glasses, i.e. which have a low melting temperature, good wettability, flowability and which seal in a gas-tight manner.
These objects and others, which will be made more apparent hereinafter, are attained by the glasses according to the invention.
According to one aspect of the present invention the lead-free bismuth-containing silicate glass has the following composition (in percent by weight, based on oxide content):
According to another aspect of the present invention the lead-free bismuth-containing silicate glass has the following composition (in percent by weight, based on oxide content):
Starting out from the composition ranges known from the above-mentioned documents, a glass was developed which, in addition to Bi2O3, contains WO3 as highly X-ray-absorbent component.
The minimum WO3 content is 0.5% by weight to achieve a sufficient effect. The maximum content is 5% by weight. In particular at high Bi2O3 content, higher levels would lead to crystalline precipitates and the glass would no longer be flowable.
The X-ray-absorbent ingredients of the glass are Bi2O3, which is present at levels between 10 and 30% by weight, preferably 15-30% by weight, and, in the highly X-ray-absorbent embodiments, between 20 and 40% by weight, and SrO, which is present at levels between 1 and 15% by weight, and, in the highly X-ray-absorbent embodiments, preferably between 1 and 10% by weight. Higher SrO content would lead to strontium silicate formation, and higher Bi2O3 content would result in precipitation of metallic bismuth.
Since the main part of the X-ray absorption is caused by three different elements, the glass is stabilized in its amorphous structure and exhibits high crystallization stability and an increased glass formation range.
The SiO2 content of the glass is at least 40% by weight and at most 60% by weight and for the highly X-ray-absorbent embodiments at least 30% by weight and at most 50% by weight. These maximum contents ensure good meltability and pressability. SiO2 serves primarily as a network former; at a content lower than the minimum content glass formation is reduced to such an extent that glasses are no longer obtained, and the chemical resistance of the glass would be reduced.
The glass contains 0.5 to 5% by weight of Al203. Al2O3 enhances the chemical resistance and the crystallization stability in the stated composition range. At lower contents the effects are too small, at higher contents the viscosity of the glass and the softening point are increased excessively, impairing the melting and processing properties of the glass.
The alkali metal oxides Na2O (1 to 10% by weight or, for the highly X-ray-absorbent embodiments, 1 to 12% by weight, preferably 1 to 10% by weight) and K2O (2 to 10% by weight or, for the highly X-ray-absorbent embodiments, 2 to 15% by weight) serve as fluxing agents and reduce the viscosity of the glass. Below the stated lower limits, the reduction in viscosity is insufficient, whereas above the stated upper limits, not only the viscosity, but also the electrical resistance is excessively reduced. In the relatively weakly X-ray-absorbent embodiments (xcexc less than 90 cmxe2x88x921 at 0.06 nm), the glass can furthermore contain up to 10% by weight of Li2O, preferably up to 5% by weight, and for the relatively highly X-ray-absorbent embodiments up to 5% by weight of Li2O, and up to 3% by weight of Cs2O.
The sum of the alkali metal oxides is between 5 and 20% by weight.
In addition to the alkaline earth metal oxide SrO, the glass also contains CaO in an amount from 0.5 to 4% by weight, and for the relatively weakly X-ray-absorbent embodiments up to 6% by weight. CaO decreases the melting temperature and increases the electrical resistance. At higher levels, the melting temperature increases excessively, and the viscosity profile as a function of the temperature is too steep, rendering the glass difficult to melt. Futhermore, the devitrification tendency increases and the flowability is restricted.
The same applies to MgO, which can be present in the glass in an amount of up to 4% by weight, and in the relatively weakly X-ray-absorbent embodiments in an amount of up to 6% by weight, and which is preferably present in an amount of at least 0.5% by weight. Partial replacement of CaO by MgO increases the devitrification stability.
The glass can furthermore contain up to 15% by weight of BaO, as X-ray-absorbent ingredient, like SrO. For the relatively highly X-ray-absorbent embodiments, up to 10% by weight of BaO are preferred. Higher contents would increase the liquidus temperature and thus the crystallization tendency.
The sum of the alkaline earth metal oxides should be between 2 and 20% by weight. For the relatively weakly X-ray-absorbent embodiments, 2 to 15% by weight are preferred.
The glass can furthermore contain conventional refining agents in the usual amounts. In particular, the glass can contain up to 0.6% by weight of is Sb2O3.
The glass can contain up to 3% by weight of ZrO2. It is preferably ZrO2-free. The glass can contain up to 2% by weight of ZnO. It is preferably ZnO-free.
The glass can furthermore contain up to 5% by weight Of MoO3 and up to 8% by weight of CeO2. These components do not only act as refining agents, but also facilitate glass formation and increase X-ray absorption. These components are therefore preferably present in the glass. The glass preferably contains at least 0.2% by weight of MoO3 and at least 0.5% by weight of CeO2.